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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
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    • CommentAuthorgravelld
    • CommentTimeMar 5th 2018
     
    I'm working on how I communicate how I want my gables extended. Currently there's a 30mm overhang, so given I want 200mm EWI (and I don't want some dodgy EWI cap) I need to extend the rafters.

    I researched "flying rafters" and "gable ladders". Most of which came up appeared to be newbuild. A few retrofit projects did come up:

    https://passivehouseplus.ie/magazine/upgrade/cork-bungalow-upgrade-phased-over-12-years

    https://springdalegarden.com/tag/gable-end/

    I kind of copied this in a concept drawing, see attached. However a couple of questions:

    1) Cutting the "noggins" (my name for them, the horizontal timber between the current last rafter and the flying rafter) into the wall seems like a lot of work, is there a way to avoid that while keeping the current roof height?

    2) Related to that, I have asked two roofers who were here on other jobs for their ideas - both independently suggested using metal (steel) instead of timber. I see little mention of this around, thoughts?
      flying_rafter.JPG
    • CommentAuthortony
    • CommentTimeMar 5th 2018
     
    Probably no need for the noggins, the battens will hold the rafter and fascia up with a presumed 30mm oversail again
    • CommentAuthorgravelld
    • CommentTimeMar 6th 2018
     
    The roofing battens that hold the membrane down??? :shocked:
    • CommentAuthoradam_w
    • CommentTimeMar 6th 2018
     
    Dan,

    Could you fit the EWI first all the way up, then clamp the Flying rafter on the outside with the use of some threaded bar fixed in to the outer leaf with a resin anchor and then shored up as Tony is suggesting? Might be a little tricky getting the resin in the hole through 200mm but it could work!
  1.  
    If as tony said "Probably no need for the noggins, the battens will hold the rafter and fascia up" with which I agree, I would ask what is the purpose of the flying rafter? I would have thought that the extended battens would be strong enough to hold a tile with a soffit fixed to the underside of the battens.
    • CommentAuthorgravelld
    • CommentTimeMar 6th 2018
     
    Well ok, let's start from first principles. Let's ignore the flying rafter; the aim is to simply extend the gable.

    Peter, can you clarify what you mean by "batten" here - which batten?

    Ah, that reminds me, I remember more detail about the "metal" solution now. One roofer was suggesting forming "n" shaped metal brackets, fixing them over the existing horizontal roofing membrane battens and having them form the overhang (so they are "hollow" underneath once they cross the existing wall).

    In these cases, is there no need to cut into the existing wall?

    Is some sort of frame running parallel to the verge to tie all the battens together not required? Just thought it might add to the strength...

    The wall is, say 100+60+100 (inner leaf + cavity + outer leaf) and the new overhang would be 250=200+10+40 (EWI+finish+overhang).
    • CommentAuthorgravelld
    • CommentTimeMar 6th 2018 edited
     
    Posted By: adam_wCould you fit the EWI first all the way up, then clamp the Flying rafter on the outside with the use of some threaded bar fixed in to the outer leaf with a resin anchor and then shored up as Tony is suggesting?
    Thanks - the flying rafter is not there for aesthetic reasons. I just thought it was the way "this is done"!



    Well ok, let's start from first principles. Let's ignore the flying rafter; the aim is to simply extend the gable.

    Peter, can you clarify what you mean by "batten" here - which batten?

    Ah, that reminds me, I remember more detail about the "metal" solution now. One roofer was suggesting forming "n" shaped metal brackets, fixing them over the existing horizontal roofing membrane battens and having them form the overhang (so they are "hollow" underneath once they cross the existing wall).

    In these cases, is there no need to cut into the existing wall?

    Is some sort of frame running parallel to the verge to tie all the battens together not required? Just thought it might add to the strength...

    The wall is, say 100+60+100 (inner leaf + cavity + outer leaf) and the new overhang would be 250=200+10+40 (EWI+finish+overhang).

    AT is interesting here in incremental retrofit...
    • CommentAuthorMarkyP
    • CommentTimeMar 6th 2018 edited
     
    I had to do this for my EWI. The joiner had extended a gable overhang it before and suggested metal box section (50mm x 25mm I think), run across two inboard rafters, recessed into the rafters flush with surface, a small chase into the blockwork, and then hang the new gable rafter from this cantilever. You could do the same with timber. In the end we did it the way Tony suggests and left the tiling battens and the sarking to take the load as we only needs to create a 150mm overhang.
  2.  
    Posted By: gravelldPeter, can you clarify what you mean by "batten" here - which batten?

    Tile battens

    Posted By: MarkyPIn the end we did it the way Tony suggests and left the tiling battens and the sarking to take the load as we only needs to create a 150mm overhang.

    It would seem that others have done this before.
    • CommentAuthorgravelld
    • CommentTimeMar 6th 2018
     
    Posted By: Peter_in_HungaryIt would seem that others have done this before.
    Sorry... who?
  3.  
    Posted By: gravelldSorry... who?

    I thought MarkyP said that
    Posted By: Peter_in_HungaryPosted By: MarkyPIn the end we did it the way Tony suggests and left the tiling battens and the sarking to take the load as we only needs to create a 150mm overhang.
    • CommentAuthorgravelld
    • CommentTimeMar 6th 2018
     
    Ok. Should I talk to a SE about whether extending the tile battens will carry the load over the distance I need, or are the calculations documented somehow?
    • CommentAuthorSilky
    • CommentTimeMar 11th 2018
     
    I bookmarked this a while back, maybe useful (basically steel extensions to fit to existing battens, available in various sizes ) https://www.dachdeckermarkt24.de/6205-dachlatten-verlaengerungen-baustatisch-geprueft.html?number=DM02428
    • CommentAuthorgravelld
    • CommentTimeMar 12th 2018
     
    Very interesting, thank you.

    In the second picture it appears to show that the timber battens are continued the "other" side of the verge. Not sure why.

    A-ha, I found a video: http://www.lemphirz.de/produkte/dachlattenverlaengerungen.html which confirms that.

    This is potentially awesome, will have to check with BC.
    • CommentAuthorwookey
    • CommentTimeMar 17th 2018
     
    I need to do this too. That lemphirz thing does look like a very neat solution. I've seen people doing this in the UK by just screwing an extra bit of batten beside the original. That would make them offset by one batten-width though so I'm not sure exactly how it was done. (I was just passing the house). There is some kind of 'plate' the tiles are set on. Not sure if it's galv steel or asbestos-a-like sheet. I need to go and take a row of tiles off for a proper look/plan. I guess you end up extending by an integer number of tiles, whatever that width is.
    • CommentAuthorgravelld
    • CommentTimeMar 17th 2018
     
    Yeah, I think extending by a round number would be expedient. Would be interested if you learn any more from that house you've seen.
    •  
      CommentAuthordjh
    • CommentTimeMar 17th 2018
     
    The lemphirz video is slightly different in that they don't fix a long batten down the ends of the new stub battens, they just fix the tiles to the stubs. So it would be more difficult to fix a verge board. I don't see what would prevent fitting such a verge batten; it might extend past the edge of the tiles though? Need to check with the manufacturer.

    The purpose of the new timber stubs is pretty obvious; it gives somewhere to use a normal fixing to fix the new tile to the extended batten. What isn't clear is whether it is 'allowed' to continue the new stub battens further than the end of the metal piece to add more than one row of tiles. i.e. what the certified use is.
    • CommentAuthorgravelld
    • CommentTimeMar 17th 2018
     
    Posted By: djhThe purpose of the new timber stubs is pretty obvious; it gives somewhere to use a normal fixing to fix the new tile to the extended batten.
    Obvious to you :wink: . Thanks!
    • CommentAuthorwookey
    • CommentTimeApr 28th 2019
     
    I just asked my EWI supplier about this and they suggested adding an extra rafter fixed to the gable end, then fix batten extension across to that. So that looks quite a lot like the pic at the head of this thread but without the tricky noggin bits. No good for a bargeboard, but fine for holding up an extra row of tiles. Main disadvantage is having to cut a bit out of the back of the EWI to go round the rafter.

    I would just buy some of those lemphirz batten-extenders, but they only do them for 50mm battens and mine are 40mm, the above supplier doesn't appear to deliver to the UK and they don;t seem to have a UK importer (I've just asked).

    There are these UK joiners:
    https://www.ebay.co.uk/itm/Roof-Batten-Joint-for-installation-of-40mm-x-25mm-1-1-2-x-1-roof-battens/302855587295
    Slightly less convincing, but I guess they would do the job so long as you attach to the existing battens far enough back for stability.

    One related matter. I have pantiles, so there is a big concrete fillet at the edge. You can see it here: http://wookware.org/pics/house/extension/html/117-image00242.jpg.html
    This has the potential to leak so unless there is some waterproofing detail there is a risk of letting water in the top of the EWI, behind the render. Anyone know a good detail for this? The cement currently sits on a 4mm thick, 150mm wide cementitious board of some type. This is presumably how it was done in the 1960s. What would one use now? I see there is something called a 'dry verge' now, which seems to be screw-on PVC edging to hold tiles down. Is that in fact better than conventional cementing?
    •  
      CommentAuthordjh
    • CommentTimeApr 28th 2019
     
    Posted By: wookeyI would just buy some of those lemphirz batten-extenders, but they only do them for 50mm battens and mine are 40mm, the above supplier doesn't appear to deliver to the UK and they don;t seem to have a UK importer (I've just asked).

    You might be able to get a local metal shop to make a functional equivalent? Or even DIY?

    Slightly less convincing, but I guess they would do the job so long as you attach to the existing battens far enough back for stability.

    Cut the existing battens back and put a new longer piece in place? Or put a length just downslope of the existing battens but extending out over the gable and then fasten a new length of batten to that?

    One related matter. I have pantiles, so there is a big concrete fillet at the edge.

    This has the potential to leak so unless there is some waterproofing detail there is a risk of letting water in the top of the EWI, behind the render. Anyone know a good detail for this?

    There's a local idiom here where the barge board is mounted alongside the tiles instead of under them and another board is laid on top of the last row of tiles. So there's effectively a long plank with an L-shaped cross-section covering the gable edge. It sounds weird but looks OK. I can't find any photos now.

    As well as the dry verge products, you can get cloaked verge tiles to use as the last ones in the row, as another alternative.
    • CommentAuthorwookey
    • CommentTimeMay 6th 2019
     
    Or put a length just downslope of the existing battens but extending out over the gable and then fasten a new length of batten to that?


    This is clearly possible, but I assume the tiles won't sit quite right if the batten is double-width.

    Or even DIY?


    I discovered my battens are 38x19mm which is pretty standard. The handy connectors which I was going to use only come in 50x25 and 40x25 so are too 'fat'. I reckon next best is just to use some 20mm galv strapping screwed to the sides of the battens at the join. I shall also bring the overhanging batten back so it's attached to at least two rafters. I reckon that will work fine.

    There's a local idiom here where the barge board is mounted alongside the tiles instead of under them and another board is laid on top of the last row of tiles. So there's effectively a long plank with an L-shaped cross-section covering the gable edge. It sounds weird but looks OK


    I guess that ends up looking a bit like this aluminium edging piece: https://www.roofingsuperstore.co.uk/product/kytun-65mm-aluminium-retrofit-tile-dry-verge-black-24m-pack-of-4.html ?

    Seems to me that should look a lot nicer than the clip-together plastic verge caps, but of course you don't get a neat seal at each tile-step - you get a gap rain can get blown into. And it doesn't really work if you have the bottom tile kicked-up (which I do).

    you can get cloaked verge tiles


    True, but not for my long-discontinued tiles, it seems.
  4.  
    Most of the dry-verge systems seem (to me) to look horrible, and a lot seem to have streaks of water-staining underneath in liner with the joints.
  5.  
    What do people think/do about the masonry leaf acting as a cold bridge, allowing 'cold' from its top edge, to flow downwards behind the EWi layer?

    Does anyone remove say the top 100mm or top course of masonry from the gable and run the insulation over the top of the gable wall, to join the EWi into the pitched roof insulation? The timbers would also run through this 100mm space to support the extended gable edge of the roof. Or is that overkill?

    Starting to research EWi as we are looking at a new project house
    •  
      CommentAuthordjh
    • CommentTimeMay 6th 2019
     
    Posted By: wookeyThis is clearly possible, but I assume the tiles won't sit quite right if the batten is double-width.

    You could always plane the extra width so the tile sits correctly, but I expect your solution will work just as well.

    I reckon next best is just to use some 20mm galv strapping screwed to the sides of the battens at the join. I shall also bring the overhanging batten back so it's attached to at least two rafters. I reckon that will work fine.

    So the join is at least two joists away from the edge? So the load on the straps will be essentially shear rather than bending? That does indeed sound like it will work.

    I guess that ends up looking a bit like this aluminium edging piece

    Like the top two sides of it anyway. I don't know how the underside is finished but I don't think there's a return, so the rain drips off clear of the wall. I'll have a closer look (with a camera) next time I see one.

    of course you don't get a neat seal at each tile-step - you get a gap rain can get blown into

    True, but I don't think that's a problem. Presumably any rain will just fall down the tiles to the eaves or get blown off the edge of the tile (under the cover) and drop down the outside of the wall. I suppose the timber is ventilated enough that it dries well.
    •  
      CommentAuthordjh
    • CommentTimeMay 6th 2019
     
    Posted By: WillInAberdeenWhat do people think/do about the masonry leaf acting as a cold bridge, allowing 'cold' from its top edge, to flow downwards behind the EWi layer?

    I agree it will act as a cold bridge. I suppose that in most cases the roof is cold, with insulation at ceiling level, so as long as there isn't a howling gale in the loft there shouldn't be huge quantities of heat lost. I suppose it depends on the ultimate goal. I think that in some of the more ambitious retrofits I've seen the solution has been to add EWI to the roof as well, so the top of the gable wall is inside the tea cosy.

    I think it's usually done by raising the roof a bit though, rather than by rebuilding the gable.
  6.  
    If it is a gable end then I would run the EWI all the way up to the ridge. If it is a gable end don't forget a significant cold bridge can occur on the inside of the gable end where the loft insulation is on the joists (cold loft). Here you can insulate the inside of the gable end to avoid this . Where the wall ends at eaves then I would join the loft insulation up with the EWI (as you suggest?) probably within the soffit. Sometimes an additional row of tiles is needed to accommodate the EWI and this simply done by extending the rafters to allow for the extra row.
  7.  
    Posted By: djh
    I agree it will act as a cold bridge. I suppose that in most cases the roof is cold, with insulation at ceiling level, so as long as there isn't a howling gale in the loft there shouldn't be huge quantities of heat lost. ....

    I think it's usually done by raising the roof a bit though, rather than by rebuilding the gable.


    Well no, if the loft space is kept full of cold outside air, then the inside face of the gable wall will be a huge cold bridge straight through the ceiling insulation, there would be no point EWI-ing the outside face of the gable wall if the inside face is left cold. As PiH suggested you could 'EWI' the inside face of the gable in the loft, but the top edge would be cold.

    The place we are considering actually has a room in the roof, so we would insulate the roof between/below the rafters, joining this onto the EWi at the eaves, the question is how to join the roof insulation to the EWI at the top of the gable?

    Unlikely to be permitted to raise the roof line to insulate on top of the existing roof structure.
  8.  
    Posted By: WillInAberdeenThe place we are considering actually has a room in the roof, so we would insulate the roof between/below the rafters, joining this onto the EWi at the eaves, the question is how to join the roof insulation to the EWI at the top of the gable?

    What options you have will depend upon whether the gable end is structural to the roof timbers. It may be possible to take down some of the inner leaf (assuming a cavity wall) and put in some CWI to join up with the roof insulation and so mitigate some of the problem or, depending upon the roof structure and the tile battens remove some of inner and outer leaf (cavity wall) or the top of the gable end (solid wall) to join up the roof insulation with the EWI.

    An alternative could be, if you can afford the space, to IWI the gable end with 100mm EPS and plasterboard over. The IWI would join up with the loft insulation.

    BTW if it is a cavity wall what is going to happen to the cavity to avoid the cavity gale nullifying the EWI?
    • CommentAuthorgravelld
    • CommentTimeMay 7th 2019 edited
     
    Posted By: WillInAberdeen
    Well no, if the loft space is kept full of cold outside air, then the inside face of the gable wall will be a huge cold bridge straight through the ceiling insulation, there would be no point EWI-ing the outside face of the gable wall if the inside face is left cold. As PiH suggested you could 'EWI' the inside face of the gable in the loft, but the top edge would be cold.
    The heat loss is proportional to the surface area exposed. It's going to be a pretty big difference between having the entire face of a wall on an elevation exposed compared to the heat path through the bridging material.

    Like djh says it depends on your goals. Not acceptable for an exacting standard, but it may still be ok. Run the numbers.

    Another thing might be the gradual knocking out of blocks and replacing them with thermally performant equivalents.

    But really the best solution is adding the roof to the tea cosy.
  9.  
    Fair enough, I'm just getting started with researching EWI as looking at our potential next project house.

    To run the numbers I need a psi-value for the thermal bridge, which I haven't found yet, does anyone know?

    As an example, single story gable wall 7m wide X 3m/6m high at eaves/ridge, EWi to u=0.1, deltaT 10oC. The direct heat loss from the room through the EWi is 7x3x0.1 X10 = 21W.

    The cold bridge from the warm room through the gable masonry into a cold loft, would be complex to calc without knowing the psi. Let's guess the average path length is 0.5m from room to loft and the gable masonry is 0.2m thick and 0.8W/mK conductivity . The heat loss would be 7m X 0.2m X 0.8W/mK X 10deg / 0.5m = 20W - so actually the cold bridge loses about as much heat as the EWI does !!

    A similar calc/result would apply for the cold bridge from a room-in-the-roof up through the gable wall to its top edge.

    So appears that if we went to the trouble of modifying the gable verge to do EWi, it would be worth fixing the cold bridge while we are up there. Also v important not to leave the gable exposed on the inside to a cold loft.

    The cold bridge is a complex 3D shape so all these numbers are very much indicative rather than accurate. Given the numbers can only guide so far, I'm interested what others have actually done in reality about this?

    Agreed it would be best to tea-cosy over the whole building but at the mo it doesn't look like we will be stripping the roof off.
   
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